1. Field of the Invention
The present invention relates to packet data transmissions, and more particularly, to a method for transmitting the packet data using a Hybrid ARQ (automatic repeat request) scheme in which packet re-transmission requests depend upon an index representative of how much the first packet data being transmitted is damaged.
2. Background of the Related Art
Generally, automatic repeat request (ARQ) and forward error correction (FEC) are well-known error control techniques commonly employed in many communication systems. In a communication system using ARQ, a receiving system makes requests to re-transmit data if the data were not received at all or were received erroneously. Therefore, an object of ARQ is to increase the reliability of the packet data transmission by re-transmitting the same data to the receiving system until the receiving system receives the data error-free. ARQ techniques provide very reliable communication, but they introduce undesirable delays, decreasing the data throughput rate of the system.
On the other hand, FEC techniques fix delay errors by using an error correction code. Therefore, an object of type 1 Hybrid ARQ schemes, in which both ARQ and FEC techniques are combined, is to have the delay error correction capability of FEC techniques and low error rates of basic ARQ scheme so that the reliability of the transmission and data throughput rates are enhanced.
In type 2 Hybrid ARQ schemes, a transmitter initially transmits a packet encoded with a given code rate. When a receiver requests to re-transmit the packet due to errors occurred, the transmitter transmits only redundant bits encoded with a lower code rate. Then the transmitted redundant bits are combined with the original data already stored in the receiver and are decoded.
FIG. 1 is a block diagram showing the structure of a xc2xd rate convolutional encoder used in a transmitter in Type 2 Hybrid ARQ according to the prior art. As it can be seen from FIG. 1, the encoder consists of a puncturing part and an encoding part having its code rate equal to xc2xd. The transmitter decreases the code rate by changing its puncturing pattern so that the redundant bits can be decoded with the lower code rate.
According to the packet transmissions in a Type 2 Hybrid ARQ scheme using a Stop And Wait (SAW) method according to the prior art, initially, a transmitter transmits a packet to a receiver. Then the receiver checks whether its buffer is empty. The fact that buffer is empty or not will decide whether the transmitted packet is self-decodable. In other words, the transmitted packet is self-decodable if the buffer is empty and is not self-decodable if it is not empty. Therefore, the receiver decodes the packet if the buffer is empty. The following step is to check whether an error has occurred. If there was no error occurred, the receiver empties its buffer and requests to send a next packet. Otherwise, it stores the packet in the buffer and requests the transmitter to re-transmit the packet.
If the buffer located in the receiver is not empty, the re-transmitted packet, which is not self-decodable, is combined with the packet(s) already stored in the buffer and is decoded.
According to the Hybrid ARQ method used in the prior art, the data throughput rate of the system is decreased and the number of the transmission errors increases if the channel environment becomes worse. Additionally, when the initially transmitted packet is severely damaged by fading, it is very difficult to be decoded after combining with the re-transmitted redundant bits since they are not self-decodable.
Accordingly, the present invention is directed to a method for transmitting packet data using a Hybrid ARQ (automatic repeat request) scheme that substantially obviates one or more problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for transmitting packet data using a Hybrid ARQ scheme in which packet data re-transmission requests depend upon an index representative of how much the packet data being transmitted for the first time is damaged so that the reliability of the transmission can be greatly improved.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method of transmitting packet data to a receiver in a communication system using a Hybrid ARQ technique includes transmitting a first packet encoded with a first code rate to a receiver, decoding the first packet in the receiver if a buffer used in the receiver is empty, comparing an index representative of how much the first packet is damaged to a predetermined threshold value if an error is occurred during the encoding process, storing the first packet in the buffer and requesting to transmit an additional packet encoded with a lower code rate if the index is greater or equal to the threshold value, requesting to re-transmit the first packet again if the index is less than the threshold value, and emptying the buffer and being ready for the next packet data transmission if no error occurred during the step of decoding the first packet.
The method further includes combining the first packet with an original packet already stored in the buffer and decoding the combined packet if the buffer is not empty, requesting to transmit the second additional packet encoded with the lower code rate if an error is occurred when the combined packet is decoded, and emptying the buffer and being ready for the next transmission if no error occurred during the step of combining and decoding.
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.